FIG. 6 shows a prior art semiconductor laser. In FIG. 6, reference numeral 1 designates an n type GaAs substrate. N type Al.sub.x Ga.sub.1-x As first cladding layer 2 is disposed on the substrate 1. Undoped Al.sub.q Ga.sub.1-q As (q&lt;x) active layer 3' is disposed on the first cladding layer 2. P type Al.sub.x Ga.sub.1-x As second cladding layer 4 having a stripe shaped ridge 10' which has a uniform width is L disposed on the active layer 3'. N type GaAs current blocking layer 8 is disposed on layer 4 except for the top surface of the stripe shaped ridge 10' of the second cladding layer 4. P type GaAs contact layer 9 is disposed on the stripe shaped ridge 10' of the second cladding layer 4 and the current blocking layer 8.
The device will operate as follows.
When a bias is applied between the p type GaAs contact layer 9 and the n type GaAs substrate 1, a thyristor is constituted by the n type first cladding layer 2, the undoped Al.sub.x Ga.sub.1-x As active layer 3', the p type Al.sub.x Ga.sub.1-x As second cladding layer 4, the n type GaAs current blocking layer 8, and the p type GaAs contact layer 9, and a current only flows through the stripe shaped convex 10' of the p type Al.sub.x Ga.sub.1-x As second cladding layer 4. The arrows 11 in FIG. 6 show the a path of this current. By that current flow, electrons and holes are injected into the undoped Al.sub.x Ga.sub.1-x As active layer 3' to and recombine radiate light. When the current is increased, laser oscillation starts. The laser light is confined, in the up and downward direction of the laser device, by the effective refractive index difference between the undoped Al.sub.x Ga.sub.1-x As active layer 3' or the n type Al.sub.x Ga.sub.1-x As first cladding layer 2 or the p type Al.sub.x Ga.sub.1-x As second cladding layer 4 and, in the transverse direction of laser device, by the guide produced by the refractive index difference between the cladding layer 4 and the n type GaAs current blocking layer 8 which absorbs light. Thus, the light is effectively guided inside the laser device.
FIG. 4 shows the relationship between the injection current and the light output in the semiconductor laser device. As shown in this figure, when the injection current is increased, destruction due to light absorption at the laser facet, that is, so-called Catastrophic Optical Damage (COD) occurs at the current injection level of point A in the figure, and the laser is destroyed. In order to enhance the power output level at which COD arises (COD level), the light density at the active layer may be decreased by reducing the thickness of the active layer or reducing the reflectance of the laser facet at the laser output side while increasing the reflectance at the opposite side, that is, with asymmetric reflection coatings However, when the active layer is made thin, the threshold current for laser oscillation rapidly increases as shown in FIG. 5 and there is a practical limit to reducing active layer thickness. Further, when the reflectance of the laser device facet is reduced, the influence of optical feedback of the emitted light from the outside is increased and the noise characteristics of the suffer. Further, by the increasing of the external differential efficiency, the light output is significantly modulated by slight variations in the injection current.
FIG. 7 shows a prior art semiconductor laser device which has an enhanced COD level and uses a superlattice active layer disordered by an impurity diffusion as disclosed in the Japanese Published Patent Application 60-101989. In FIG. 7, reference numeral 1 designates an n type GaAs substrate. N type Al.sub.x Ga.sub.1-x As first cladding layer 2 is disposed on the n type GaAs substrate 1. Superlattice layer 3 comprising N+1 GaAs quantum well layers and N Al.sub.y Ga.sub.1-y As barrier layers is disposed on the first cladding layer 2. P type Al.sub.x Ga.sub.1-x As second cladding layer 4 is disposed on the superlattice layer 3. P type GaAs contact layer 5 is disposed on the second cladding layer 4. Reference numeral 13 designates the neighborhood of the cavity facet which has become a uniform mixed crystal due to the Zn diffusion. Reference numerals 14 and 15 designates a p side and an n side electrode, respectively.
The production process will be described.
First of all, an n type Al.sub.x Ga.sub.1-x As first cladding layer 2, a superlattice layer 3, a p type Al.sub.x Ga.sub.1-x As second cladding layer 4, and a p type GaAs contact layer 5 are successively epitaxially grown on a GaAs substrate 1. Thereafter, a silicon nitride film or a silicon oxide film is deposited on the entire surface of wafer and this film is patterned to produce a plurality of stripe shaped apertures along the region becoming the cleavage planes. Next, using the patterned film as a mask, Zn is diffused from the stripe shaped apertures. The superlattice layer 3 inside the region 13 into which Zn is diffused is disordered, and it becomes a Al.sub.y 'Ga.sub.1-y 'As mixed crystalline layer having a larger band gap than the transition energy between the bottom levels of the quantum well of the superlattice 3. Thereafter, the wafer is cleaved into chips at the neighborhood of the central portion thereof along the stripes of the apertures.
In this laser device, a multi-quantum, well structure is used as an active layer and by increasing the energy band gap only the neighborhood of the facet of the active layer, the, light absorption loss at that portion is lowered, thereby increasing the COD level.
In this prior art device, the COD levels can be increased without lowering the light density at the active layer by thinning the active layer or by lowering the light density inside the laser device by asymmetric reflection coating which lowers the reflectance of cavity facet at the light output side and increases the same at the opposite side thereof. Accordingly, the above-described problems can be solved.
The prior art laser device thus constructed has increased the COD level by disordering the superlattice active layer with an impurity diffusion In this prior art laser device, however, the light confinement in the transverse direction inside the active layer is not considered. As the light confinement in the transverse direction inside the active layer, the loss guide type shown in the prior art of FIG. 6 is thought of. However, in a case where the construction of FIG. 6 is combined with the construction of FIG. 7, the production process becomes quite difficult because an increased number of photolithography steps are required Further, in producing the electrodes, the mask which becomes a diffusion mask in the production process of the device of FIG. 7 must be removed. When the diffusion mask is removed, it is quite difficult to distinguish the diffusion region from the other region and to determine the positions to be cleaved.